In order to provide for accurate mating of outside threads to fasteners with inside threads, outside screw threads must be meticulously measured for uniformity. The "pitch" of a screw thread is the distance between a point on a screw thread and the corresponding point on the adjacent thread in a line parallel with the axis of the screw. Moreover, the "pitch diameter" of a screw is the diameter of a screw measured from a midpoint on the flank surface of a screw thread through the screw to an opposite midpoint on a flank of the screw thread of the screw, in a direction perpendicular to the axis of the screw. In other words, on a straight thread, the pitch diameter is the diameter of the cylinder whose surface passes through the thread profiles at such points as to make the widths of thread groove and thread ridge equal.
Various methods have been provided for measuring screw threads by using the aforementioned three wires for measuring the pitch diameter of the screw threads. Screw threads are measured in accordance with United States Department of Commerce, National Bureau of Standards HANDBOOK H28, entitled "Screw-Thread Standards For Federal Services-Appendix For Methods of Wire Measurement of Pitch Diameter of 60 degree Threads". According to the federal standards, the measurement of the pitch diameter, that is the distance between the centers of two adjacent screw threads, the measurement must be accurate with no room for movement of the wires, which wires are placed upon the flank surfaces within the screw threads for the purpose of measuring the pitch diameter.
It is further noted that while the aforementioned appendix applies to 60 degree threads, other angled threads are similarly measured.
In a typical use, one hardened steel longitudinally extending center wire is placed upon the flank surfaces within the tapered walls of the screw thread. On the opposite side of the screw thread to this center wire are two offset wires placed upon the flank surfaces in the corresponding tapered walls between the threads of the screw.
Typically, the hardened steel longitudinally extending cylinder wires are placed in the thread groove. The two offset wires are placed on one side of the screw and the one single center wire is placed on the opposite side.
In the prior art, various holders are described for holding the wires within a micrometer anvil so that one holder, such as that having the two offset wires, is advanced towards the opposite holder with the center wire.
As noted above, the wires should be placed such that the outside circumference of the cylindrical wire touches exactly the mid slope of the flank of the thread of a given pitch. However, in the prior art, in order to hold the wires exactly in the proper measuring position, two movable holding members, such as rubber bands or springs, are typically used to hold the wires still in place during the measurement of the screw threads. However, use of any "movable" holders, such as rubber bands or springs, is not allowed in government standards as well as in standard engineering texts, such as the Machinery Handbook, which notes that the three measuring wires should be firmly held in place during measurement.
When standard micrometers are used, however, often the use of the three-wire method for measuring screw threads is an unwieldy manual process, which reduces the repeatability of measurement, in which the user must hold the micrometer in one hand, as well as the three wires, while the other hand moves the handle, while moving the anvils of the micrometer, to position the double offset wires and the single center wire. This movement enables the wires to make contact with the thread groove.
Various devices have been patented to attempt to use the three-wire method for measuring screw threads. Among these are U.S. Pat. Nos. 2,431,021 of Bourdelais, 2,939,220 of Croshier, 2,822,624 of Klink, 2,692,438 of Schneider, 2,906,029 also of Croshier, 2,962,817 of Varney, 3,812,591 of Michaud, 4,480,388 of O'Brien, 4,553,337 of Brewster, 4,590,678 of Arredondo, 4,777,361 of Affa, 4,821,422 of Porter, 4,939,845 also of Porter, 4,947,555 of Allen and 5,175,935 of Welter.
The Bourdelais '021, Croshier '220, Klink '624, Schneider '438, Croshier '029, Barney '817, O'Brien '388, Porter '422, Porter '845 and Welter '935 patents generally disclose wire holding attachments for the three-wire method of measuring pitch diameters of screw threads. However, some of the known devices have significant disadvantages, such as the fact that the devices cannot hold the three wires firmly in place, within the screw threads, because of the use of springs which, as the springs are compressed smaller, results in the likelihood that there is increasing pressure upon the three wires, which may result in the possibility of the dislodging of the wires.
Such use of springs is disclosed in the Croshier '220 and '029 patents. In Croshier '220, two off-set wires, which are held in place against the screw threads, are movable, which can cause inaccuracies of the measurements. Likewise, in Croshier '029, the wires are held apart by a hub, so that the wire pins float, and are not locked in a solid position against the flank surfaces within the screw threads.
Furthermore, in Croshier '220, the holes holding the wires are larger than the diameter of the cylindrical wires, which may cause the wires to shift while conforming to the flank surfaces of the screw threads. The shifting may result in inaccurate measurements and is therefore contraindicated according to the aforementioned Federal standards and engineering handbooks.
Moreover, O'Brien '388 requires a flexible holder, such as a rubber band, which is not permitted under the federal Handbook H28.
Many of the prior art patents have complicated moving parts and springs, which do not provide for accurate measurement of screw threads. Another problem with the prior art devices, such as O'Brien '388, is that they do not have a positive means for the wires to adjust to the lead angle of the threads being measured.
The Michaud '591 Patent describes a ridged gage tip and the Brewster '337 Patent discloses the use of a control screw thread to measure a screw thread, but it has the disadvantage that it may move too much in place for accurate measurement. The Allen '555 Patent describes rotating measuring balls instead of longitudinally extending straight wires, which makes it difficult to locate the exact center of the screw diameter. Affa '361 describes a housing with a measuring scanning unit, and Arredondo '678 discloses use of a single wire instead of three wires to measure threads. Moreover, since the outer side of the screw thread opposite to the single wire is held against a flat surface, the measurement is inaccurate because if the outside diameter of a screw thread varies, the pitch diameter measurement will be inaccurate.
In an unpatented prior art publication, there is shown a three wire measuring system in the catalog of the Mahr Gage Co., Inc. of 274 Lafayette St., New York, N.Y., which defines a calibration device which includes a pair of holder ear pieces for holding the respective center wire and two off-set wires against the screw threads at opposite sides of the screw threads being measured. In the Mahr system, the two sets of wires move in place as the non-rotating spindle of a micrometer is advanced towards the other wire holder with the single center wire.
The disadvantage of the Mahr device is that the measuring wires are advanced towards each other in a horizontal orientation, thereby resulting in the possibility of inaccuracies, because of the effect of gravity upon the horizontally oriented holder ear pieces for the wires. In the Mahr device the wires are solely held in place against gravity in a horizontal position by the wire holder ears. Moreover, in the Mahr device the wires are held in place in one location within the holder ear pieces, so that if a particular wire tends to wear out where it is being used to measure screw thread pitch diameter on a frequent basis, the particular wire cannot be moved in an axial direction so as to expose a further portion of the wire, which is not worn, out to the screw threads.